This invention is directed to wire connectors and more specifically to so-called screw-on or twist-on wire connectors which are more economical to produce.
Twist-on wire connectors are widely used in the electrical trades for joining electrical wires. Twist-on connectors are adapted to be screwed down on the stripped ends of electrical wires. These types of connectors typically have a shell or cap made of an insulating material, such as plastic. The shell is shaped for easy manipulation. For example, the exterior surface of the shell may have raised ribs, protrusions or wings to facilitate gripping and/or turning. The exterior surface may also include features to reduce slippage when twisting the connectors on by hand. The exterior may further be adapted to accommodate the use of hand tools, such as wrenches and pliers, or tool chucks so that power tools could be used to perform the installation.
The interior surface of the shell typically has a generally conically-shaped bore or cavity with a larger opening at the point of entry of the stripped electrical wires. The cavity tapers towards a closed end. Seated within the shell""s cavity is a coiled metal wire commonly referred to as a spring. The outer contour of the spring usually matches the conically-shaped interior surface of the shell. Thus, the spring has a larger opening at the electrical wire receiving end and a smaller opening adjacent the closed end portion of the shell. The spring also defines an internal cavity which receives the electrical wires therein. Examples of prior art twist-on wire connectors are shown in U.S. Pat. Nos. 4,220,811, 4,227,040 and 4,691,079, the disclosures of which are incorporated herein by reference.
The spring facilitates the holding or gripping of the stripped ends of electrical wires in order to maintain contact between the electrical wires. The spring also grips the interior surface of the shell to prevent it from becoming detached and leaving the bare electrical wires exposed.
The cross-sectional shape of the wire used to form the spring is important to providing secure engagement with the stripped electrical wire ends and the interior surface of the shell. Presently, the most common cross-sectional shape is square or rectangular. The spring is oriented in the shell so that one comer or crest of this cross-sectional shape extends toward the inner surface of the shell. When the spring is inserted into the shell during manufacture of the connector the crest will grip the inner surface of the shell securely, thereby locking the spring and shell together. The opposite corner or crest of the spring wire""s cross-sectional shape faces the cavity of the spring, positioning this crest for engagement with an electrical wire inserted into the connector. In this manner, each winding or coil of the spring presents a crest which can bite into the stripped ends of the wires in order to provide adequate fixation of the connector to the electrical wires.
Such wire connectors and the metallic spring inserts have not changed much over the years. The changes that have occurred focused on trying to improve the holding ability of the spring such as by changing the shape of the spring from a conical helix to an hourglass shaped helix or by increasing the number of engaging edges of the spring. However, these changes usually resulted in increasing the cost of manufacturing the wire connector. Therefore, there is a need for a twist-on wire connector that is more economical to produce without jeopardizing its gripping and holding ability.
The most expensive component of twist-on wire connectors is the metallic spring insert. One way to decrease the cost of the wire connector would be to reduce the amount of material used to make the spring. The reduction can be accomplished by decreasing the size of the spring or reducing the number of windings or coils of the helical spring. The spring could also be made thinner. However, all these approaches would also degrade the ability of the connector to grip and stay attached to the wires since less engagement surface will be available. Therefore, with this invention, it has been determined that the overall cost of a twist-on wire connector can be reduced by changing the cross-sectional shape of the wire used to make the spring to eliminate material not necessary to the proper functioning of the wire connector.
A twist-on wire connector includes an insulating cap having a body wall and an end wall. The body wall is typically conical and has an inner surface defining a generally conically-shaped internal cavity. There is an opening at one end of the shell cavity. A spring is mounted within the shell cavity and is fixedly attached to the inner surface of the body wall. The spring has a plurality of coils which in one embodiment have a hexagonal shaped cross-section. The lateral surfaces of the hexagonal shape may be planar or concave. In an alternate cross-sectional shape of the spring coils, there is an internal bore or void in the coils.
It has been determined that by changing the cross-sectional shape of the wire used to make the spring as described, an economically significant reduction in the amount of material used to make the spring can be realized without any loss of gripping and holding capacity. Holding and gripping is not compromised because the new cross-sectional shape eliminates portions of the old cross-sectional shape which have an insignificant effect on the gripping and holding ability of the spring with respect to both the stripped electrical wires and the interior surface of the shell.